T-TWR Rev 2.1 FSK Digital Modem
A 1000 bps FSK software modem for the LILYGO T-TWR Rev 2.1, enabling digital data transmission over UHF with LDPC forward error correction and real-time ASCII waterfall display.
Overview
This project implements a pure software FSK modem for the LILYGO T-TWR Rev 2.1, a programmable UHF walkie-talkie covering 400–480 MHz. Written by HB9HUH/K6CFT, it achieves up to 1000 bps data throughput using 4-FSK (500 Baud) or 2-FSK (600 Baud) modulation, with error-free transmission demonstrated beyond one mile out of the box.
Technical Implementation
The firmware leverages the ESP32-S3's Sigma-Delta modulation peripheral to generate phase-continuous FSK audio for transmission, and uses hardware ADC sampling at 6400 Hz for reception. A Fast Hartley Transform (FHT) analyzes the received spectrum in real time, and a symbol-level synchronization clock is derived from FHT data. The UHF band covered includes European PMR license-free channels and the IARU-R1 70 cm amateur radio band.
Features and Error Correction
Variable-length packets up to 128 bytes are supported. Configurable LDPC forward error correction (256, 512, or 1024 parity bits at code rates 4/5, 2/3, or 1/2) uses the ESP32 CPU for decoding. Framing uses 64-bit CCSDS Codeblock Sync Markers and a CCSDS randomizer. A 12-bit CRC provides additional integrity checking. An echo-request service with GPS timestamp and location enables field range testing. Sent and received messages are persisted to on-device flash and inspectable via the serial CLI.
